Don't get obsessed with "there should be 230v down the other neutral",,, there isn't, you've proved it already.
Some Possibles.
1st neutral failed some time ago, 2nd failed recently.
The ring isn't what it claims to be and both neutrals from the C.U go to a J.B or socket/spur then off to the ring, but have failed at that point.
The 2 neutrals "connected" in the C.U are bad connections.